What it means
Markets are forward looking, which means today's price already contains a consensus view of the future. News moves prices only to the extent it revises that view, which is why a company can announce record profits and see its shares fall if the market expected even more.
The effect is usually measured as an abnormal return, meaning the part of a day's price move that cannot be explained by the market's own movement. Isolating it requires stripping out the return you would expect given the market's direction and the share's sensitivity to it.
Timing is important because expectations build before the event. Prices often drift in the anticipated direction for days beforehand, so analysts look at a window around the announcement rather than the single day, and add the abnormal returns across it.
Not every announcement produces a lasting effect. A genuine change in expected cash flows tends to stick, while a move driven by surprise or sentiment can reverse within days as investors work through the detail.
The lesson for managers is that the market reacts to the gap between news and expectation, not to the news alone. Careful expectation management, meaning guidance that can realistically be met, is often worth more than a single impressive number.
In practice
Real-world examples.
Example
A pharmaceutical company reports that a trial met its primary endpoint and the shares jump 22% in a session, adding roughly $600,000,000 of market value years before any product revenue arrives. The price is reacting to a change in probability, not to cash received.
Example
A retailer announces earnings 4% above consensus but cuts its full-year outlook, and the shares fall 8%. The beat was already priced in while the downgrade was not, which is the announcement effect working on expectations rather than on results.
Example
A central bank signals that rate rises are likely to pause, and government bond yields fall the same afternoon even though no rate has changed. Markets have repriced the expected path of rates purely on the strength of the wording.
Formula
Calculation
Abnormal return = actual return - expected return, where expected return = beta x market return
Cumulative abnormal return = the sum of daily abnormal returns across the event window
A company announces a major contract win. On the announcement day its shares rise 6.4% while the market index rises 1.2%, and the share has a beta of 1.1.
Expected return = 1.1 x 1.2% = 1.32%, so the abnormal return is 6.4% - 1.32% = 5.08%. With a market capitalisation of $800,000,000 before the news, that abnormal move represents 5.08% x $800,000,000 = $40,640,000 of value attributed to the announcement itself.
Widening the window, the shares showed a +0.4% abnormal return the day before as rumours circulated and a -0.6% abnormal return the day after as investors reassessed the margin on the contract. The cumulative abnormal return over the three days is 0.4% + 5.08% - 0.6% = 4.88%.Case study
Seen in the real world.
This is an illustrative, entirely fictional case. Pellhaven Engineering, an invented industrial supplier with a market capitalisation of $800,000,000, announced a long-term contract with a national infrastructure operator.
On the day, the shares rose 6.4% against a market up 1.2%. With a beta of 1.1, the expected return was 1.32%, so the abnormal return was 5.08%, worth about $40,640,000 of value created by the news alone. The board treated this as the market's verdict on the deal.
The fuller picture emerged over three days. The shares had already risen 0.4% abnormally the day before as the tender result leaked, and fell 0.6% abnormally the day after when analysts modelled the contract's thin first-year margin, giving a cumulative abnormal return of 4.88%. In this fictional case the finance director began publishing expected margin ranges alongside contract wins, having learnt that the second-day reassessment was where credibility was won or lost.
Watch out
Common mistakes.
- Assuming good news must lift a share price, when the price only moves on the difference between the news and what was already expected.
- Reading the raw daily price move as the announcement effect without stripping out the market's own movement.
- Judging a reaction on a single day, when leaks beforehand and reassessment afterwards are often where the real story sits.
Questions
People also ask.
Why did our shares fall on a record profit?
Because the market had already priced in a stronger result, so the announcement was a downgrade relative to expectations.
How long does an announcement effect last?
News that genuinely changes expected cash flows tends to hold, while a sentiment-driven move often fades within days.
Can a company manage the effect?
Yes, largely through guidance and disclosure that keep expectations realistic, so results land close to what investors already believe.
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